The present invention relates to a spring-type pretensioner for a vehicle seat belt system, and in particular to a trigger for operating such a pretensioner by releasing its preloaded spring.
Vehicle seat belt systems are often provided with pretensioners, which pull the belt firmly against the body of an occupant in an emergency. There are various types of pretensioners, such as ones that rotate the belt retractor reel in the belt-winding direction, ones that directly pull the belt such as by forming a loop in it, and ones that pull on the buckle stalk. As the driving source for such operation, gunpowder is often used.
When gunpowder is used as the driving source, an electric trigger is required to operate it, resulting in an increased cost. To avoid this problem, spring-type pretensioners, which are purely mechanical devices that use a spring as the driving source, have been proposed.
Regardless of the type of pretensioner, the driving source for operating the pretensioner must have sufficient operating force and stroke to complete the belt-tensioning operation within a very short time after a collision of the vehicle. For this reason, spring-type pretensioners require a powerful spring held with a very large preload. To hold the preload, it is necessary to have a trigger mechanism that provides a large operating force and an acceleration detecting sensor that produces a mechanical signal to initiate the operation of the trigger mechanism to release the spring preload.
Mechanical trigger mechanisms and sensors have been disclosed in the Japanese Provisional Utility Model Publications No. 90159/1982 and No. 33060/1983 (Japanese Utility Model Publication No. 35017/1988), Japanese Provisional Patent Publications No. 206765/1983, No. 164650/1989, and No. 164651/1989. All of these devices are based on arrangements in which the restraint of the spring is released upon movement of an inertia body that directly restrains a trigger mechanism.
In the conventional arrangements as described in the aforementioned references, it is necessary to have a large inertia body to obtain sufficient sensor output and a lever with a long arm to amplify the sensor output to provide a large operating force. The large size and weight of these mechanisms associated with the pretensioner are undesirable.